How do fabric softener sheets work in reducing static electricity?

In fabric softener there is a water-soluble polymer that acts as a anti static during the rinse cycle. Fabric sheets are basically, designed sheets with fabric softener added to them for the dryer.
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Fabric softeners work by coating the surface of the cloth fibers with a thin layer of chemicals; these chemicals have lubricant properties and are electrically conductive, thus making the fibers feel smoother and preventing buildup of static electricity.

Cationic softeners bind by electrostatic attraction to the negatively charged groups on the surface of the fibers and neutralizing their charge; the long aliphatic chains are then oriented towards the outside of the fiber, imparting lubricity. Vinegar works on some materials in a similar way, as the hydrogen ions bind to the anionic groups on the fibers.


https://en.wikipedia.org/wiki/Fabric_softener